Air flow rate measureing apparatus

ABSTRACT

An object of the present invention is to provide an air flow rate measuring apparatus of a heat generating resistor type in which it is possible to adequately take the advantage of the air flow rate measuring apparatus of a heat generating resistor type and to obtain a high control precision when the air flow rate measuring apparatus is applied to the engine control.  
     A flow rate signal V with a nonlinear characteristic generated from a flow rate detecting unit  3  provided with a heat generating resistor  3   a,  is converted into a signal V 1  with a linear characteristic by a linearizing circuit  4.  Then, the flow rate signal is smoothed by a filter circuit  5  to obtain a signal V 2  in which measurement error due to a flow rate ripple is suppressed. Thereafter, the flow rate signal is again formed into a signal V 3  with a nonlinear characteristic by a nonlinear-form converting circuit  6,  whereby resolution lowering due to analog-to-digital conversion can be suppressed.  
     According to the above arrangement, it is possible to prevent the output voltage from being lowered due to the nonlinear characteristic of the heat generating resistor and the ripple caused from an engine. Further, the resolution of the analog-to-digital converter is prevented from being lowered when the engine control unit is supplied with flow rate signal. As a result, it is possible to obtain a stable output even if the air flow rate stays in a low rate region such as when the engine is driven in a region of idling speed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for measuring anair flow rate by using a heat generating resistor, and more particularlyto an air flow rate measuring apparatus advantageously utilized forcontrolling the air-fuel ratio of an internal combustion engine of anvehicle.

[0003] 2. Description of the Related Art

[0004] It has been well known that one of apparatus for measuring a flowrate of a fluid such as air is a flow rate measuring apparatus of a heatgenerating resistor type.

[0005] The flow rate measuring apparatus of the heat generating resistortype is utilized in such a manner that the heat generating resistorsupplied with energy electically heated is provided in a fluid of whichflow rate is under measurement. Then, the flowing velocity is detectedby using that the quantity of heat taken away from the heat generatingresistor by the fluid becomes a function of the flowing velocity,whereby the flow rate is measured.

[0006] Recently, because of the advantages that will be described lateron, the flow rate measuring apparatus of the heat generating resistortype has been widely utilized for measuring an intake air flow rate forthe air-fuel ratio control of an engine (internal combustion engine) ofa vehicle.

[0007] According to the flow rate measuring apparatus of the heatgenerating resistor type, it is possible to measure the mass flow ratedirectly. For this reason, there can be obtained advantages that dataobtained therefrom need not be corrected based on the atmosphericpressure or temperature.

[0008] Further, the flow rate measuring apparatus of the heat generatingresistor type has a nonlinear characteristic such that the relationshipbetween the flow rate and the detected signal exhibits a steep slope inthe low flow rate region and the slope becomes gentler as the flow rateis increased. For this reason, it is possible to obtain a widemeasurement range in which an error rate need not be differently set.Moreover, even if the flow rate is small with the result that the flowrate exhibits only a small change, it is possible to obtain an outputvoltage change large enough to be detected by an analog-to-digitalconverter with a reasonable resolution. Accordingly, the flow ratemeasuring apparatus of the heat generating resistor type is extremelyadvantageous in application to a control of idling engine speed forstabilizing the same.

[0009] Conversely, due to the nonlinear characteristic, the flow ratemeasuring apparatus requires a correction processing for linearizing theoutput. For this reason, in a prior art, as for example shown in FIG.11, a flow rate signal V generated from a flow rate detecting unit 3 issupplied to an engine control unit 2 in which the flow rate signal V issubjected to a linearizing processing and also subjected to an averagingprocessing by a filter upon necessity. Thus, data indicative of an airflow rate Q is obtained.

[0010] The engine control unit 2 calculates a fuel injection amount foran engine under consideration of other parameters such as a throttlevalve opening degree, an engine speed as shown in the figure. Thus, theair-fuel ratio for the engine is controlled for the engine not shown.

[0011] The flow rate detecting unit 3 is composed of a heat generatingresistor 3 a as a detecting element. The heat generating resistor 3 a isprovided within an air passage A such as an intake manifold of theengine or the like so that the heat generating resistor 3 a is exposedto an intake air flow AF.

[0012] On the other hand, there has been proposed a prior artarrangement of the flow rate measuring apparatus of the heat generatingresistor type in which, as shown in FIG. 12, influence of backward flowcaused from an intake air ripple of the engine within the intakemanifold is corrected.

[0013] According to the prior art arrangement, as shown in FIG. 12, aflow rate detecting unit 30 is provided with, in addition to theoriginal heat generating resistor 3 a, another heat generating resistor3 b serving for detecting the backward flow of intake air. With thisarrangement, data indicative of an air flow rate Q having been subjectedto the backward flow correction can be obtained.

[0014] A prior art relating to this kind of correction can be found inJapanese Patent Laid-Open No. Hei 8-94406, for example.

[0015] The above-described prior art, however, does not take intoaccount deterioration in detection precision due to the averagingprocessing which is carried out by a filter in addition to thelinearizing processing on the flow rate signal. Thus, the prior art willencounter the following difficulties.

[0016] As described above, the flow rate measuring apparatus of the heatgenerating resistor type has some advantages and disadvantages.

[0017] One of advantages is that the mass flow amount can be directlymeasured, the data obtained therefrom need not be corrected based on thechange in atmospheric pressure or temperature. Another advantage is thatthe flow rate measuring apparatus of the heat generating resistor typehas a nonlinear characteristic that the relationship between the flowrate and the output voltage exhibits a steep slope in a low flow rateregion while a gentle slope in a high flow rate region. Therefore, evenwhen the flow rate is low and thus it exhibits small flow ratefluctuation such as when the engine is placed in an idling drive mode,it is possible to obtain an output voltage fluctuation amount largeenough to be detected by an analog-to-digital converter with areasonable resolution, which fact is useful for stabilizing the idlingspeed of the engine.

[0018] Conversely, the flow rate measuring apparatus of the heatgenerating resistor type has a drawback that the nonlinearcharacteristic causes a measurement error.

[0019] This phenomenon is caused because the relationship between theflow rate and the output voltage is not linear, with the result that themean value of the output voltage is decreased with respect to the meanvalue of the flow rate due to the engine speed ripple or the like.

[0020] This phenomenon particularly acts on increase in the air-fuelratio (the ratio of fuel to air is decreased) in view of the enginecontrol standpoint, leading to decrease in output of the engine.

[0021] Now, the drawback of the prior art will hereinafter be described.

[0022]FIGS. 13 and 14 show the relationship between the ripple amplitudeand the detected voltage of the heat generating resistor of the flowrate measuring apparatus of the heat generating resistor type accordingto the prior art arrangement shown in FIG. 11.

[0023] These diagrams are characteristic diagrams in which the air flowrate Q is plotted in abscissa while the voltage value V of the detectedsignal detected by the heat generating resistor 3 a is plotted inordinate. In this case, FIG. 13 is a characteristic diagram in which theengine control unit does not carry out the averaging processing with ahard filter while FIG. 14 is a characteristic diagram in which theengine control unit carries out the averaging processing with a hardfilter or the like.

[0024] Since air flowing the intake manifold of the engine ripples withthe opening and closing motion of the intake valve, the detected airflow rate is also rippled as shown in the figures.

[0025] Since the temperature of the heat generating resistor 3 asubstantially faithfully responds to the fluctuation of the air flowrate, the detected signal also ripples as shown in the figures.

[0026] At this time, since the voltage value characteristic of thedetected signal detected by the heat generating resistor 3 a relative tothe air flow rate exhibits nonlinearity as shown in the figure, the meanvalue of the detected signal is decreased relative to the mean value ofthe original air flow rate Q, which fact leads to a measurement errorderived from the nonlinearity of the heat generating resistor and therippled amplitude.

[0027] However, if the heat generating resistor could detect the intakeair ripple without any response delay, the detected ripple could besequentially converted into the air flow rate, and the mean valuethereof could be calculated, then the decrease in the means value asdescribe above should be avoided regardless of the deviation in the meanvalue of the detected signal.

[0028] However, even if the intake air ripple can be detected withoutany response delay by the heat generating resistor, it is very difficultto convert the intake air ripple component into the air flow ratecontinuously. Therefore, it is almost impossible in view of practicalstandpoint, since the air flow measuring apparatus of the heatgenerating resistor type is installed in an engine room of a vehicle,and the air flow rate measuring apparatus suffers from ignition noise orintake air disturbance which cause error in detected data. Further, toomuch calculation task can be imposed on a signal processing unit of theengine control unit.

[0029] For this reason, the prior art arrangement of the air flow ratemeasuring apparatus of the heat generating resistor type employs a hardfilter for averaging the output voltage when the output voltage issupplied to an analog-to-digital converter in the engine control unit 2.A filter selected as the hard filter is ordinarily one composed of aresistor and a capacitor having a time constant of 1 to 20 ms.

[0030] As set forth above, FIG. 14 shows a characteristic of the airflow rate measuring apparatus when the apparatus employs a hard filter.Since the hard filter acts on the characteristic such that the amplitudeis made small without changing the mean value of the output voltagevalue. Therefore, if the detected value is directly converted into theair flow rate, an error that the mean value of the converted air flowrate is also decreased is caused, resulting in a characteristicdifferent from that shown in FIG. 13. In this case, however fast theanalog-to-digital converter carries out the sampling operation, errorwill be caused in the detected flow rate value.

[0031] Recently, there are many cases in which two heat generatingresistors are employed to detect the backward flow of air generated inthe intake manifold and data is corrected based on the detected backwardflow amount, whereby detecting precision of the flow rate measuringapparatus is increased. In this case, however, small backward flow willcause a large fluctuation in the output voltage. Therefore, averagingoperation on the value of the output voltage gives an excessive backwardflow amount compensation.

[0032] The above-mentioned drawback can be avoided by carrying out datasampling at a high rate and linearizing processing continuously.However, in order to avoid the drawback, it is necessary to carry outthe processing at a sampling rate of at least 1 ms. Therefore, theengine control unit will be loaded with a heavy duty such as a highsampling speed, a large arithmetic operation task or the like that willnot be requested when other type of sensors are employed.

[0033] In detail, the prior art arrangement shown in FIG. 12distinguishes the direction of air flow in such a manner that when thedetected output voltage V is larger than the threshold value Vm (airflow ratio=0), the direction of the air flow is determined forward whilewhen the detected output voltage V is smaller than the threshold valueof Vm, the direction of the air flow is determined backward. In thiscase, similarly to the prior art shown in FIG. 11, if the prior artarrangement converts the output value into the air flow ratecontinuously without using a hard filter, the characteristic thereofbecomes as shown in FIG. 15. At this time, as far as the mean value ofthe output voltage is concerned, measurement error excessive withrespect to that in the case of FIG. 12 is caused. However, after theoutput voltage is converted into the air flow rate continuously,measurement value does not contain error.

[0034] However, if the output voltage is converted into the air flowrate by way of a hard filter, as shown in FIG. 16, measurement error isleft unremoved and it becomes impossible to detect the backward flow.Therefore, the original purpose, i.e., to detect backward flow cannot beaccomplished.

[0035] As described above, the air flow rate measuring apparatus of theheat generating resistor type provides not only advantages but alsodisadvantages due to its nonlinear characteristic, and hence it isdifficult to take only an advantage thereof at the current stage, whichfact is recognized as a problem to be solved.

SUMMARY OF THE INVENTION

[0036] Therefore, an object of the present invention is to provide anair flow rate measuring apparatus of a heat generating resistor type inwhich it is possible to adequately take the advantage of the air flowrate measuring apparatus of a heat generating resistor type and toobtain a high control precision when the air flow rate measuringapparatus of the heat generating resistor type is applied to the enginecontrol.

[0037] In order to attain the above object, there is provided an airflow rate measuring apparatus having a flow rate detecting unit which isutilized for measuring air flow rate using a heat generating resistorprovided in an air flow passage, including a linearizing circuitinputted with a signal indicative of a flow rate having a nonlinearcharacteristic from the flow rate detecting unit, a filter circuitinputted with a signal supplied from the linearizing circuit, and anonlinear-form converting circuit inputted with a signal supplied fromthe filter circuit, wherein the output of the nonlinear-form convertingcircuit is extracted as a flow rate detecting signal.

[0038] Further objects and advantages of the present invention will beapparent from the following description which is given with reference tothe accompanying drawings wherein preferred embodiments of the presentinvention is clearly shown.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a block diagram showing one embodiment of an air flowrate measuring apparatus of a heat generating resistor type according tothe present invention;

[0040]FIG. 2 is a characteristic diagram useful for explainingresolution of the linear characteristic and nonlinear characteristic onthe relationship between the output voltage of the heat generatingresistor and the idling flow rate in the one embodiment of the presentinvention;

[0041]FIG. 3 is a block diagram showing another embodiment of an airflow rate measuring apparatus of a heat generating resistor typeaccording to the present invention;

[0042]FIG. 4 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor due to backward flow, aripple amplitude and the measurement error due to a filter;

[0043]FIG. 5 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor, a ripple amplitude and themeasurement error due to a filter;

[0044]FIG. 6 is a block diagram showing one example of a linearizingcircuit of the one embodiment of the present invention;

[0045]FIG. 7 is a block diagram showing one example of a nonlinear-formconverting circuit of the one embodiment of the present invention;

[0046]FIG. 8 is a circuit diagram showing one example of a flow ratedetecting unit of the one embodiment of the present invention;

[0047]FIG. 9 is a front view showing one example of a flow ratedetecting unit of the one embodiment of the present invention;

[0048]FIG. 10 is a side cross-sectional view showing one example of theflow rate detecting unit of the one embodiment of the present invention;

[0049]FIG. 11 is a block diagram showing one example of an air flow ratemeasuring apparatus of a heat generating resistor type according to aprior art technology;

[0050]FIG. 12 is a block diagram showing one example of an air flow ratemeasuring apparatus of a heat generating resistor type with a backwardflow detecting system according to a prior art technology;

[0051]FIG. 13 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor, a ripple amplitude and themeasurement error due to the filter;

[0052]FIG. 14 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor, a ripple amplitude and themeasurement error due to the filter;

[0053]FIG. 15 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor of the backward flowdetecting system, a ripple amplitude and the measurement error due tothe filter; and

[0054]FIG. 16 is a characteristic diagram useful for explaining thenonlinearity of the heat generating resistor of the backward flowdetecting system, a ripple amplitude and the measurement error due tothe filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] An embodiment of the air flow rate measuring apparatus of a heatgenerating resistor type according to the present invention willhereinafter be described in detail with reference to the drawings.

[0056] Initially, the principle on which the air flow rate measuringapparatus of the heat generating resistor type will be described.

[0057]FIG. 8 is a circuit diagram of a flow rate detecting unit 3 of oneexample of the air flow rate measuring apparatus of the heat generatingresistor type. The detecting unit 3 is mainly composed of a bridgecircuit and a feedback circuit.

[0058] The bridge circuit is formed of a heat generating resistor 3 aprovided within an air flow passage A and a temperature sensing resistor3 c also provided within the air flow passage A and useful for intakeair temperature compensation, and resistors 3 d and 3 e for constitutingthe bridge circuit.

[0059] The feedback circuit is formed of an operational amplifier OP1and a transistor Tr.

[0060] The operational amplifier OP1 detects an unbalanced voltage fromthe bridge circuit and controls the current supplied through thetransistor Tr to the bridge circuit based on the unbalanced voltage,whereby a feedback is effected on the circuit.

[0061] Owing to the feedback, a heating current Ih flowing through theheat generating resistor 3 a is controlled so that temperaturedifference is constantly maintained between the heat generating resistor3 a and the temperature sensing resistor 3 c. As a result, the heatingcurrent Ih is varied depending on the air flow rate.

[0062] That is, when the flow rate of the intake air flow AF is large,the amount of heat taken away from the heat generating resistor 3 abecomes large, with the result that the amount of the heating current Ihis increased. When the flow rate of the intake air flow AF is small, theamount of heat taken away from the heat generating resistor 3 a becomessmall, with the result that the amount of the heating current Ih isdecreased. As a result, the value of the heating current Ih becomesproportional to the flow rate of the intake air flow AF. Then theheating current Ih is converted into a voltage by the resistor 3 d sothat an output signal V indicative of an air flow rate Q is obtained.

[0063]FIG. 9 is a frontal illustration as viewed from the upstream sideof the air passage of one example of an air flow rate meter of the heatgenerating resistor type. FIG. 10 is a side cross-sectional view of thesame. Thus, FIG. 9 is obtained when the air flow rate measuringapparatus is viewed from the left side in FIG. 10.

[0064] The air flow rate meter illustrated in these figures is mainlycomposed of a conduit member 20 forming a part of the air flow passageA, a housing member 22 in which a circuit board 21 is provided, asupport member 23 made of a dielectric material, and a sub-air flowpassage member 24.

[0065] The sub-air flow passage member 24 has two support bodies 25 madeof a conductive wire material provided in the sub-air flow passagemember 24. The support bodies 25 are provided with the heat generatingresistor 3 a and the temperature sensing resistor 3 c and electricallyconnected to the circuit board 21. The whole assembly except for theconduit member 20 constitutes an integral module of the air flow ratemeter.

[0066] The heat generating resistor 3 a and the temperature sensingresistor 3 c may be formed of a substrate made of glass, ceramic,silicon or the like covered with a thin film or thick film of platinum,tungsten or the like. The resultant parts can be utilized as the heatgenerating resistor and the temperature sensing resistor.

[0067] For example, a diaphragm is formed by using silicon, and the heatgenerating resistor is provided on the diaphragm.

[0068] The conduit member 20 is provided with an aperture 26 on itswall. The sub-air flow passage member 24 is inserted into the aperture26, and the housing member 22 is attached to the conduit member 20 bymeans of screws 27, whereby the air flow rate meter is arranged suchthat the inner space of the conduit member 20 is utilized as a main airflow passage 28 and inner space of the sub-air flow passage member 24 isutilized as a sub-air flow passage 29.

[0069] In this arrangement, the conduit member 20 is provided with asealing member 31 such as an O ring at the aperture 26 so thatairtightness is maintained.

[0070] Now, one embodiment of the present invention will be describedwith reference to FIG. 1.

[0071] In FIG. 1, reference numeral 1 depicts an overall arrangement ofthe air flow rate measuring apparatus of the heat generating resistortype.

[0072] In the arrangement of the air flow rate measuring apparatus ofthe heat generating resistor type 1, reference numeral 4 depicts alinearizing circuit, 5 a filter circuit and 6 a nonlinear-formconverting circuit. The rest of the arrangement is the same as that ofthe prior art arrangement described with reference to FIG. 11.

[0073] The main circuits constituting the air flow rate measuringapparatus of the heat generating resistor type 1 are mounted on thecircuit board 21 shown in FIGS. 9 and 10.

[0074] As for example shown in FIG. 6, the linearizing circuit 4 isformed of two-stage multiplying circuits connected to each other in acascade fashion. The linearizing circuit 4 is supplied with the flowrate signal V generated from the heat generating resistor 3 a, andserves as a converter which converts the flow rate signal V having acharacteristic, i.e., a nonlinear characteristic with respect to the airflow rate Q into a signal having a linear characteristic with respect tothe air flow rate.

[0075] It is known that the relationship between the flow rate signal Vgenerated from the heat generating resistor 3 a and the air flow rate Qis such that fourth power of the flow rate signal V is proportional tothe air flow rate Q.

[0076] Thus, if the flow rate signal V is subjected to two-stagemultiplying circuits connected in a cascade fashion, the flow ratesignal V can be almost linearized. Thus, the linearizing circuit 4 canoutput a linearized flow rate signal V1.

[0077] The filter circuit 5 is formed of a CR hard filter composed of aresistor and a capacitor so as to have a time constant of 1 to 20 ms,for example. The filter circuit 5 has the function of suppressing theamplitude of the flow rate signal V1 without changing the mean valuethereof. With this function, the filter circuit 5 generates a flow ratesignal V2 having been subjected to the averaging operation.

[0078] As for example shown in FIG. 7, the nonlinear-form convertingcircuit 6 is formed of one operational amplifier and two multiplyingcircuits, supplied with the flow rate signal V2 generated from thefilter circuit 5, and carries out a processing to convert the flow ratesignal V2 into a signal having a predetermined nonlinear characteristic,i.e., a nonlinear characteristic opposite to the characteristic of thelinearizing circuit 4. That is, the nonlinear-form converting circuit 6converts the flow rate signal V2 which has been subjected to thelinearizing processing in the linearizing circuit 4 into a flow ratesignal V3 with a nonlinear characteristic which is the same as thecharacteristic of the signal having not been subjected to thelinearizing processing.

[0079] In the nonlinear-form converting circuit 6 shown in FIG. 7, theinput signal passing through the two-stage multiplying circuit issubtracted from the input signal by the operational amplifier so that anonlinear characteristic opposite to the characteristic of thelinearizing circuit 4 is obtained. As shown in the figure, if thenon-inverted input signal Vin=V2 of the operational amplifier is denotedwith a symbol X, the output of the operational amplifier is X1, theoutput of the first stage multiplying circuit of the operationalamplifier is X2, and the output of the second stage multiplying circuitof the operational amplifier is X3, then the relationship among them canbe expressed as follows.

X2=X1^ 2

X3=X2^ 2=X1^ 4

X3=X

X=X1^ 4

∴x1=Vout=fourth root of (X)

[0080] Thus, it is possible to obtain the nonlinear-form convertingcircuit 6 having a fourth root characteristic opposite to the fourthpower characteristic of the linearizing circuit 4.

[0081] The engine control unit 2 takes the flow rate signal V3 intoitself and linearizes the flow rate signal V3 to produce an air flowrate Q, similarly to the prior art technology. Further, the enginecontrol unit 2 is supplied with a throttle valve opening degree signal,an engine speed signal or the like. Then, the engine control unit 2calculates a fuel injection amount necessary for maintaining the enginecombustion condition optimum based on these signals, and supplies theresultant signal to a fuel injection valve of the engine as a fuelinjection amount signal.

[0082] Now, the operation of the embodiment will be described.

[0083] According to the above-described embodiment, the flow rate signalV generated from the flow rate detecting unit 3 is subjected to thelinearizing processing in the linearizing circuit 4, also subjected tothe averaging processing in the filter circuit 5 formed of the hardfilter, and then the flow rate signal V2 is obtained.

[0084] In any cases, the detection error of the above-described priorart technology results from the operation that the output voltage valueobtained from the heat generating resistor is subjected to the averagingprocessing in the hard filter and thereafter the resultant signal isconverted into a signal indicative of the air flow. Therefore, if, as inthe present embodiment, the detecting signal detected from the heatgenerating resistor 3 a having a high speed response characteristic isinitially subjected to the linearizing processing in the linearizingcircuit 4 and thereafter subjected to the averaging in the filtercircuit 5, the error inherent in the above-described prior arttechnology will be eliminated.

[0085] Accordingly, if the problem to be solved by the present inventionwas only the error inherent in the above-described prior art technology,it should be sufficient to form an arrangement that the output of theflow rate detecting unit 3 is supplied to the linearizing circuit 4 tobe subjected to the linearizing processing and then supplied to thefilter circuit 5 to be averaged.

[0086] However, the above-arrangement conversely will cause a problemdescribed as follows. For this reason, the embodiment of the presentinvention is further provided with a nonlinear-form converting circuit6, as shown in the figure.

[0087] That is, a problem raised in this case is an input resolution ofan analog-to-digital converter upon low flow rate such as when theengine is placed in an idling mode.

[0088] Although not shown in the figure, the analog-to-digital converteris provided within the engine control unit 2, and functions to convertthe supplied flow rate signal into a digital signal. A generalanalog-to-digital converter employed in a vehicle is one having an inputresolution of ten bits. In such a case, an input voltage extending in afull scale is divided into 1024 unit portions.

[0089] On the other hand, an input voltage to be processed by amicrocomputer for use in a vehicle extends in a range of from 0.0 V to5.12 V. In this case, the minimum resolution becomes about 5 mV.

[0090] Now, if the detected signal is not subjected to the linearizingprocessing and left unprocessed to be a signal with a nonlinearcharacteristic N as shown in FIG. 2, the change amount of the outputvoltage corresponding to the change amount of ΔQ in the air flow rate Qbecomes ΔV_(N), which is large enough in terms of resolution. Therefore,no problem will be caused. This is one of remarkable features of the airflow rate measuring apparatus, as described above.

[0091] However, if the detected signal is subjected to the linearizingprocessing to become a signal with a linear characteristic L, i.e., thesignal passes through only the linearizing circuit 4 and the filtercircuit 5, then, the voltage change amount of the detected signalcorresponding to the same flow rate change amount ΔQ becomes ΔV_(L),which is far smaller as compared with the change amount ΔV_(N) obtainedwhen the detected signal is left unprocessed to be a signal with anonlinear characteristic.

[0092] How much change amount in the output voltage will be obtainedwith respect to the air flow rate change of 1% amount is examined undera condition that the engine is substantially driven within a range of anidling speed. If the detected signal is converted into a signal with anonlinear characteristic N as shown in FIG. 2, then a detected voltagechange amount of about 5 mV is obtained. This value is large enough tobe detected as the air flow change by an analog-to-digital converterhaving ten bits resolution. Therefore, it is possible to utilize thedetection result for engine control.

[0093] Conversely, if the detected signal is processed to have a linearcharacteristic L, the obtained change amount is merely about 1 mV, whichis too small to be detected as the air flow rate. As a result, if thedetection result is applied to the engine control, the engine controlbecomes unstable when the engine is driven at the idling speed.

[0094] For this reason, the embodiment shown in FIG. 1 is provided withthe nonlinear-form converting circuit 6, by which the flow rate signalV2 with the linear characteristic L generated from the linearizingcircuit 4 is processed to have a nonlinear characteristic. Thus, theflow rate signal is again formed into the flow rate signal V3 having thenonlinear characteristic N which is the same as the signal before thestage of processing in the linearizing circuit 4, and then supplied tothe engine control unit 2.

[0095] Therefore, according to the embodiment of FIG. 1, it is possibleto adequately suppress the lowering phenomenon in the output from theheat generating resistor 3 a caused from the ripple of the amplitude.Accordingly, it is possible to positively eliminate the fear that thechange amount of the output voltage becomes smaller than the resolutionof the analog-to-digital converter.

[0096] Moreover, the analog-to-digital converter on the side of theengine control unit 2 need not carry out sampling operation at a highrate. Therefore, to lighten the load of arithmetic operation will notincrease error in measurement of the air flow.

[0097] At this time, according to the prior art technology, when themean air flow rate is calculated, the output signal obtained from theair flow rate measuring apparatus should be sequentially converted intothe air flow rate within the engine control unit. However, according tothe present embodiment, the air flow rate can be calculated from themean value of the output voltage of the air flow rate measuringapparatus of the heat generating resistor type.

[0098] In other words, according to the embodiment, on the side of theengine control unit, it is allowable to apply a hard filter having alarge time constant which is permissible in terms of engine controlprior to the processing by the analog-to-digital converter. Owing tothis fact, the load imposed on the engine control unit 2 can beremarkably decreased.

[0099] Further, according to the present embodiment, the air flow ratemeasuring apparatus is tough against noise caused from ignition or thelike. Therefore, error caused from noise can be satisfactorilysuppressed and the engine can be controlled with high precision.

[0100] The reason is that when the air flow rate measuring apparatus isapplied to control of an engine of a vehicle and the air flow ratesignal is supplied from the air flow rate detecting unit to the controlunit, if influence of noise on other equipment is considered, it isconsiderably advantageous to supply a signal with a nonlinearcharacteristic to an interface of the control unit for controlling theflow rate in view of S/N standpoint.

[0101] Another embodiment of the present invention will hereinafter bedescribed.

[0102]FIG. 3 shows another embodiment of the present invention in whichthe overall arrangement thereof is fundamentally the same as that of theembodiment shown in FIG. 1 but the arrangement shown in FIG. 3 isreplaced at the flow rate detecting unit 3 with a flow rate detectingunit 30. The flow rate detecting unit 30 includes two heat generatingresistors 3 a, 3 b in the flow rate detecting unit 3. The two heatgenerating resistors 3 a, 3 b are utilized for detecting backward flowby heat transaction between them and correcting the detected data. Thedetection style thereof the same as that of the prior art technologyshown in FIG. 12.

[0103] According to the embodiment shown in FIG. 3, the outputcharacteristic of the detecting unit 30 is such that the output rangecorresponding to the forward flow and the output range corresponding tothe backward flow are divided at a threshold value Vm of the outputvoltage, as shown in FIG. 4. For this reason, the change amount ΔVcorresponding to the air flow change amount ΔQ which might be small whenthe engine is driven at an idling speed, becomes considerably small ascompared with the change amount ΔV shown in FIG. 5 detected by thedetecting unit 3 which dose not detect backward flow. Therefore, if nocountermeasure is taken, the idling speed becomes unstable.

[0104] However, according to the embodiment shown in FIG. 3, a flow ratesignal V′ that is an output from the flow rate detecting unit 30 havingthe heat generating resistors 3 a, 3 b is once linearized in alinearizing circuit 40 and thereafter sent to a filter circuit 50 inwhich a ripple amplitude is suppressed until a signal V2′ containing nobackward flow component is obtained without changing the mean value ofthe waveform of the flow rate signal V1′ containing a backward flowcomponent.

[0105] In this way, since the flow rate signal is formed into a ripplewaveform containing no backward flow component, the output voltage rangeneed not be divided into the range of forward flow and the range ofbackward flow at a threshold value Vm, as shown in FIG. 4. Therefore,when the flow rate signal is converted into a signal with a nonlinearcharacteristic in a nonlinear-form converting circuit 60, the signal canbe converted into one extending within a range as shown in FIG. 5 inwhich only the forward flow is to be considered.

[0106] Therefore, also according to the embodiment of FIG. 3, it isneedless to say that the effect obtained by the embodiment shown in FIG.1 can be expected. Moreover, when the embodiment shown in FIG. 3 isapplied for controlling an engine, it is possible to avoid the alreadydescribed instability phenomenon in the idling speed caused from thatthe change amount of the output voltage becomes smaller than the inputresolution of the analog-to-digital converter. Furthermore, it ispossible to satisfactorily decrease the measurement error due to thebackward flow.

[0107] Incidentally, according to the above embodiments, either of theflow rate signals V3, V3′ generated from the air flow measuringapparatus of the heat generating resistor type is made into a signalwith a nonlinear characteristic. Therefore, these signals are apparentlythe same as the flow rate signal V of the prior art technology shown inFIGS. 11 and 12.

[0108] Therefore, according to the above embodiments, when the air flowmeasuring apparatus is applied for controlling an engine of a vehicle,the apparatus can be dealt in the same manner as that of the air flowmeasuring apparatus of the heat generating resistor type. Accordingly,the air flow measuring apparatus of the heat generating resistor typeaccording to the present invention offers satisfactory compatibilitywith that of the prior art technology when it is applied to an enginecontrol unit.

[0109] According to the present embodiment, it is possible to preventthe output voltage from being lowered due to the nonlinearcharacteristic of the heat generating resistor and the intake air rippleof the engine. Further, it is possible to keep the change amount of theoutput voltage supplied to the engine control unit larger than theresolution of the analog-to-digital converter. As a result, when the airflow measuring apparatus of the heat generating resistor type accordingto the present invention is applied for controlling an engine, it ispossible to obtain a precise and stable control with ease even if theair flow rate stays in a low rate region such as when the engine isdriven in a region of idling speed.

[0110] Although several embodiments have been described above, theseembodiments are merely illustrative and not restrictive. Therefore, itis apparent to those skilled in the art that various changes andmodifications can be effected without departing from the spirit andscope of the invention, and thus these changes or modifications shouldbe embraced within the spirit and scope of the claims appended hereto.

What is claimed is:
 1. An air flow rate measuring apparatus having aflow rate detecting unit which is utilized for measuring air flow rateusing a heat generating resistor provided in an air flow passage,comprising: a linearizing circuit supplied with a signal from the flowrate detecting unit; a filter circuit supplied with a signal from thelinearizing circuit; and a nonlinear-form converting circuit suppliedwith a signal from the filter circuit; wherein the output of thenonlinear-form converting circuit is extracted as a flow rate detectingsignal.
 2. An air flow rate measuring apparatus according to claim 1,wherein the flow rate detecting unit is provided with two resistors sothat the flow rate detecting unit can detect backward flow.
 3. An airflow rate measuring apparatus according to claim 1, wherein thenonlinear-form converting circuit is formed of a circuit for convertingits input signal into a signal having a characteristic which exhibits asteep slope in the low flow rate region while for converting its inputsignal into a signal having a characteristic which exhibits a gentleslope in the high flow rate region.
 4. An air flow rate measuringapparatus according to claim 1, wherein the linearizing circuit carriesout linear approximation on the relationship between the input signaland the flow rate, and the nonlinear-form converting circuit carries outcurve approximation on the relationship between the input signal and theflow rate.